Gradient immersion lithography

ABSTRACT

In a lithographic projection apparatus, a space between an optical element is filled with a first fluid and a second fluid separated by a transparent plate. The first and second fluids have first and second indices of refraction, respectively, that are different from one another. The first fluid is provided between a substrate and the transparent plate and has an index of refraction similar to the index of refraction of the substrate. The second fluid is provided between the transparent plate and the optical element and has an index of refraction similar to the index of refraction of the optical element. The transparent plate has a third index of refraction between the first and second indices of refraction and may be equal to the first index of refraction or the second index of refraction. A device manufacturing method includes filling a space between the optical element and the substrate with at least two fluids having different indices of refraction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic apparatus, in particular to a lithographic apparatus in which the space between an optical element of a projection system and a substrate is filled with at least two fluids having different indices of refraction. The present invention also relates to a device manufacturing method.

2. Description of the Related Art

The term “patterning device” as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device. An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase shift, and attenuated phase shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.

Another example of a patterning device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.

Another example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.

For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.

Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step and scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the beam of radiation in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.

In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement and/or inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemical, mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is important to ensure that the overlay (juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as “alignment system”), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.

For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.” Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,796.

In order to increase the degree of integration of semiconductor devices and keep pace with Moore's law, it will be necessary to provide lithographic projection apparatus capable of printing practical minimum line widths of 25-100 nm. Currently available photolithographic tools using 193 nm and 157 nm radiation can produce pattern features having a resolution (in nm) according to the well known equation R=k₁·λ/NA, where R is the resolution, k₁ is a constant that is dependent on the radiation sensitive material (resist) used, λ is the wavelength of the radiation, and NA is the numerical aperture. The numerical aperture NA is determined according to the equation NA=n·sin θ, where n is the index of refraction of the material through which the radiation passes and θ is the angle of incidence of the radiation. A lower limit of k₁ is 0.25 and lithographic projection apparatus having a NA of 0.85 are currently available. Difficulties in optical design make increasing the NA difficult. As k₁ and NA are generally considered to be at their limits, the ability to decrease the resolution, i.e, print smaller pattern features, of lithographic projection apparatus appears to be dependent on decreasing the wavelength λ of the radiation.

One method of decreasing the wavelength of the radiation is to immerse the substrate in a lithographic projection apparatus in a fluid having a relatively high refractive index, e.g., water, so as to fill the space between the optical element of the projection system (lens) and the substrate. Filling the space between the optical element of the projection system and the substrate enables the imaging of smaller features as the exposure radiation has a shorter wavelength in the fluid. The effect of the fluid may be regarded as increasing the effective NA of the system. This method can also be used to increase the depth of focus for imaging at the same resolution. The depth of focus is defined by the equation DOF=+/−k₂ λNA², where k₂ is a process dependent constant, λ is wavelength, and NA is numerical aperture. Depth of focus is one of the factors determining the resolution of the lithographic apparatus and is defined as the distance along the optical axis over which the image of the pattern is adequately sharp.

One difficulty presented by filling the space between the optical element of the projection system and the substrate with a fluid is the fluid has an index of refraction between the indices of refraction of the optical element and the substrate. The index of refraction of the fluid may not be sufficient to improve the resolution of the image. The fluid is only able to provide a single decrease in the wavelength of the radiation as the index of refraction of the fluid is constant throughout the space between the optical element and the substrate.

Another problem associated with immersion lithography is that many fluids are insufficiently transparent to 193 nm and 157 nm radiation which would tend to decrease the amount of radiation incident on the substrate.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide a lithographic apparatus in which the space between the optical element of the projection system is filled with at least two fluids having different indices of refraction in order to increase the angle of incidence of the radiation on the substrate, thus reducing reflection of the radiation by the wafer and increasing the resolution of the apparatus. As used herein, the term fluid refers to liquids, gases, and/or gels.

This and other aspects are achieved according to the invention in a lithographic apparatus including a radiation system constructed and arranged to supply a beam of radiation; a support constructed and arranged to support a patterning device, the patterning device constructed and arranged to pattern the beam of radiation according to a desired pattern; a substrate table to hold a substrate; a projection system constructed and arranged to project the patterned beam onto a target portion of the substrate; a transparent plate positioned between an optical element of the projection system and the substrate; a first fluid having a first index of refraction filling a first space between the substrate and the transparent plate; and a second fluid having a second index of refraction filling a second space between the transparent plate and the optical element.

According to a further aspect of the invention there is provided a device manufacturing method including projecting a patterned beam of radiation onto a target portion of a layer of radiation-sensitive material at least partially covering substrate using a projection system; and filling a space between an optical element of the projection system and the substrate with at least two fluids, wherein the two fluids have different indices of refraction.

Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid crystal display panels, thin film magnetic heads, etc. n will appreciate that, in the context of such alternative applications, any use of the terms “reticle”, “wafer” or “die” in this text should be considered as being replaced by the more general terms “mask”, “substrate” and “target portion”, respectively.

In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which:

FIG. 1 is a schematic illustration of a lithographic projection apparatus according to an examplary embodiment of the invention;

FIG. 2 is a schematic illustration of immersion of the substrate table, the substrate and the optical element of the projection system of the apparatus of FIG. 1;

FIG. 3 is a schematic illustration of the passage of a patterned beam of radiation from an optical element through a second fluid, a transparent plate, and first fluid incident on a substrate;

FIG. 4 is a schematic illustration of a system for providing fluid between the projection system and the transparent plate;

FIG. 5 is an enlarged view of the system of FIG. 4; and

FIG. 6 is a schematic illustration of a system for providing fluid between the transparent plate and the substrate.

In the Figures, corresponding reference symbols indicate corresponding parts.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic projection apparatus 1 according to an embodiment of the invention. The apparatus includes a radiation system Ex, IL constructed and arranged to supply a beam PB of radiation (e.g. UV or EUV radiation, such as, for example, generated by an excimer laser operating at a wavelength of 248 nm, 193 nm or 157 nm, or by a laser-fired plasma source operating at 13.6 nm). In this embodiment, the radiation system also comprises a radiation source LA. The apparatus also includes a first object (mask) table MT provided with a mask holder constructed and arranged to hold a mask MA (e.g. a reticle), and connected to a first positioning device PM to accurately position the mask with respect to a projection system or lens PL; a second object (substrate) table WT provided with a substrate holder constructed and arranged to hold a substrate W (e.g. a resist-coated silicon wafer), and connected to a second positioning device PW to accurately position the substrate with respect to the projection system or lens PL. The projection system or lens PL (e.g. a quartz and/or CaF₂ lens system or a refractive or catadioptric system, a mirror group or an array of field deflectors) is constructed and arranged to image an irradiated portion of the mask MA onto a target portion C (e.g. comprising one or more dies) of the substrate W. The projection system PL is supported on a reference frame RF. The apparatus may employ another kind of patterning device, such as a programmable mirror array of a type as referred to above.

The source LA (e.g. a UV excimer laser, an undulator or wiggler provided around the path of an electron beam in a storage ring or synchrotron, a laser-produced plasma source, a discharge source or an electron or ion beam source) produces radiation. The radiation is fed into an illumination system (illuminator) IL, either directly or after having traversed a conditioner, such as a beam expander Ex, for example. The illuminator IL may comprise an adjusting device AM for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in the beam. In addition, it will generally comprise various other components, such as an integrator IN and a condenser CO. In this way, the beam PB impinging on the mask MA has a desired uniformity and intensity distribution in its cross-section.

It should be noted with regard to FIG. 1 that the source LA may be within the housing of the lithographic projection apparatus (as is often the case when the source LA is a mercury lamp, for example), but that it may also be remote from the lithographic projection apparatus, the radiation beam which it produces being led into the apparatus (e.g. with the aid of suitable directing mirrors). The latter scenario is often the case when the source LA is an excimer laser. The present invention encompasses both of these scenarios. In particular, the present invention encompasses embodiments wherein the radiation system Ex, IL is adapted to supply a beam of radiation having a wavelength of less than about 170 nm, such as with wavelengths of 157 nm, 126 nm and 13.6 nm, for example.

The beam PB subsequently intercepts the mask MA, which is held on the mask table MT. Having traversed the mask MA, the beam PB passes through the lens PL, which focuses the beam PB onto a target portion C of the substrate W. With the aid of the second positioning device PW and interferometer IF, the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB. Similarly, the first positioning device PM can be used to accurately position the mask MA with respect to the path of the beam PB, e.g. after mechanical retrieval of the mask MA from a mask library, or during a scan. In general, movement of the object tables MT, WT will be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning). However, in the case of a wafer stepper (as opposed to a step and scan apparatus) the mask table MT may just be connected to a short stroke actuator, or may be fixed. The mask MA and the substrate W may be aligned using mask alignment marks M₁, M₂ and substrate alignment marks P₁, P₂.

The depicted apparatus can be used in two different modes:

-   1. In step mode, the mask table MT is kept essentially stationary,     and an entire mask image is projected at once, i.e. a single     “flash,” onto a target portion C. The substrate table WT is then     shifted in the X and/or Y directions so that a different target     portion C can be irradiated by the beam PB; -   2. In scan mode, essentially the same scenario applies, except that     a given target portion C is not exposed in a single “flash.”     Instead, the mask table MT is movable in a given direction (the     so-called “scan direction”, e.g. the Y direction) with a speed v, so     that the beam PB is caused to scan over a mask image. Concurrently,     the substrate table WT is simultaneously moved in the same or     opposite direction at a speed V=Mv, in which M is the magnification     of the lens PL (typically, M=¼ or ⅕). In this manner, a relatively     large target portion C can be exposed, without having to compromise     on resolution.

Referring to FIG. 2, the substrate table WT is immersed in a first fluid 10 having a first index of refraction. The first fluid 10 is provided by a first fluid supply system 15. A transparent plate, or dish, 12 is positioned between the projection system PL and the substrate table WT. The space between an optical element OE (e.g., a lens) of the projection system PL and the transparent plate 12 is filled with a second fluid 11 having a second index of refraction. The second index of refraction is different from the first index of refraction. The second fluid 11 is provided by a second fluid supply system 16. The transparent plate 12 has a third index of refraction that is preferably between the first index of refraction of the first fluid 10 and the second index of refraction of the second fluid 11.

Referring to FIG. 3, an optical element OE of the projection system PL has a index of refraction n_(OE) and the substrate W has an index of refraction n_(W) that is larger than the index of refraction n_(OE) of the optical element OE. The optical element, for example a lens, may be formed, for example, of glass, fused silica, quartz, or CaF2, and have, for example a index of refraction n_(OE) of about 1.4 to about 1.6. The optical element OE may be formed of, for example, fused silica, commonly used in deep ultraviolet (DUV) lithography, and have an index of refraction n_(OE) of about 1.46. As another example, the optical element OE may be formed of CaF2, commonly used in 157 nm lithography, and have an index of refraction n_(OE) ranging from about 1.4 to about 1.56.

The second fluid 11 may be, for example, water and have an index of refraction n_(SL) of about 1.3 to 1.35 for use with radiation having a wavelength of 248 nm. In the case of using water as the second fluid 11 with radiation having a wavelength of, for example 193 nm, the index of refraction n_(SL) should be approximately 1.44. The index of refraction n_(W) of the substrate W may be, for example, between 1.7 to 1.8.

The first fluid 10 has an index of refraction n_(FL) that may be relatively higher than the index of refraction n_(SL) of the second fluid 11. The first fluid 10 may be a perfluoropolyether (PFPE) fluid. The PFPE fluid may be, for example, FOMBLIN® or GALDEN®, available from Ausimont Corporation, or KRYTOX®, available from DuPont. PFPE fluids are currently used as lubricants in vacuum pumps and thus are compatible with clean room environments in which photolithographic projection apparatus are used. PFPE fluids are optically clean, non-toxic, chemically inert, and compatible with at least some current resist materials. PFPE fluids have a 157 nm absorbance α=10⁻³ μm⁻¹ base 10, which is a thousands times lower than current experimental 157 nm resists. FOMBLIN® Z-25 has an index of refraction at 157 nm of n=1.35. FOMBLIN® Y-18 has an index of refraction at 157 nm of n=1.37. FOMBLIN® Y-140, a higher molecular weight version, has an index of refraction at 157 nm of n=1.38.

PFPE fluids also have an index of refraction that is closely matched to CaF₂, commonly used to form optical elements of the projection system in 193 nm lithography for color correction and commonly used for optical elements in 157 nm lithography. PFPE fluids are chemical and solvent resistant. They also have excellent thermal and electrical resistance and are non-reactive with metal, plastic, elastomers and rubber. PFPE fluids are inert to liquid and gaseous oxygen and are nonflammable. Because PFPE fluids are inert to liquid and gaseous oxygen they are not affected by the high oxygen conditions found in photo-resist stripping processes. PFPE fluids can also withstand Lewis acids produced during aluminum etching, products from sulfur, most acids, most bases and most oxidizing agents. They are available in a variety of viscosities and have low evaporation loss. PFPE fluids also have excellent radiation hardness and resistance to polymerization in the presence of ionizing radiation. PFPE fluids have zero ozone depletion potential and are not classified as volatile organic chemicals by the Environmental Protection Agency.

The transparent plate 12 has an index of refraction n_(TP) that is between the index of refraction n_(FL) of the first fluid 10 and the index of refraction n_(SL) of the second fluid 11. The transparent plate 12 may be formed of any material that is transparent to the particular wavelength radiation, e.g., 365 nm, 248 nm, 193 nm, 157, or 126 nm, generated by the radiation system Ex, IL of the lithographic projection apparatus 1 and has an index of refraction n_(TP) between the indices of refraction of the first fluid 10 and the second fluid 11, including an index of refraction n_(TP) being equal to either the index of refraction n_(FL) or n_(SL) of the first fluid 10 or the second fluid 11, respectively. It should also be appreciated that the entire plate 12 need not be transparent, only those portions through which the patterned projection beam passes through.

As shown in FIG. 3, the patterned beam PPB exits the optical element OE of the projection system PL and enters the second fluid 11. As the patterned beam PPB passes the boundary between the optical element OE and the second fluid 11 and passes into the second fluid 11, it is deflected as its velocity changes upon entering the second fluid 111. It should be appreciated that if the index of refraction n_(SL) of the second fluid 11 is equal to the index of refraction n_(OE) of the optical element OE, the patterned beam PPB will not be deflected.

The transparent plate 12 may have an index of refraction n_(TP) that is equal to or higher than the index of refraction n_(SL) of the second fluid 11. As the patterned beam PPB passes from the second fluid 11 into the transparent plate 12 it is deflected. It should be appreciated that the index of refraction n_(TP) of the transparent plate 12 may be equal to the index of refraction n_(SL) of the second fluid 11 in which case the patterned beam PPB will not be deflected upon entering the transparent plate 12.

The first fluid 10 has an index of refraction n_(FL) that may be equal to or higher than the index of refraction n_(TP) of the transparent plate 12. If the index of refraction n_(TP) of the transparent plate 12 is equal to the index of refraction n_(SL) of the second fluid 11, the index of refraction n_(FL) of the first fluid 10 must be higher than the index of refraction n_(TP) of the transparent plate 12 to provide a variable index of refraction. If the index of refraction n_(FL) of the first fluid 10 is higher than the index of refraction n_(TP) of the transparent plate 12, the patterned beam PPB is deflected as it leaves the transparent plate 12 and enters the first fluid 10. The patterned beam PPB passes through the first fluid 10 and is then incident on the substrate W.

Although the first fluid 10 has been described as a PFPE fluid and the second fluid 11 has been described as water, it will be appreciated that the first and second fluids 10 and 11 may both be PFPE fluids, or any other fluid of suitable transparency to the wavelength of radiation provided by the radiation system of the apparatus, as long as the index of refraction n_(FL) of the first fluid 10 is different than the index of refraction n_(SL) of the second fluid 11. It should also be appreciated that the first and second fluids 10 and 11 are selected based on their indices of refraction relative to the indices of refraction of the substrate W and the optical element OE of the projection system PL, respectively. The first fluid 10 should have an index of refraction similar to the index of refraction of the substrate and the second fluid should have an index of refraction similar to the index of refraction of the optical element as possible.

The index of refraction from the optical element, to the second fluid, to the translucent plate, to the second fluid, and to the wafer is thus continuously varied. Any combination of indices of refraction of the optical element, the second fluid, the translucent plate, the first fluid and the substrate that produces a variable index of refraction between the optical element and substrate so as to increase the angle of incidence of the patterned projection beam on the substrate is contemplated by the invention. The effect of this continuously varying index of refraction is an increase in the angle of incidence of the patterned projection beam on the substrate, a decrease of reflection of the patterned projection beam by the substrate, and an increase in the resolution of the apparatus.

It should be appreciated that more than two fluids may be provided between the optical element and the substrate. The space may be filled with three or more fluids. It should also be appreciated that the three or more fluids may be separated by two or more translucent plates, respectively. It should also be appreciated that the depth of the first and second fluids may be varied (e.g. by the fluid supplies 15 and/or 16) to further vary the index of refraction from the optical element to the substrate. It should further be appreciated that the depth (thickness) of the translucent plate may be varied to further vary the index of refraction from the optical element to the substrate.

FIG. 4 shows the second fluid 11 between the projection system PL and the transparent plate 12. A fluid reservoir 20 is filled with the second fluid 11 provided via inlet/outlet ports 23.

The reservoir 20 forms a contactless seal to the transparent plate 12 around the image field of the projection system PL so that fluid is confined to fill the space between the transparent plate 12 and the optical element of the projection system PL. The reservoir 20 is formed by a seal member 21 positioned below and surrounding the optical element of the projection system PL. The second fluid 11 is brought into the space below the projection system PL and within the seal member 21. The seal member 21 extends a little above the optical element of the projection system PL and the fluid level rises above the optical element so that a buffer of fluid is provided. The seal member 21 has an inner periphery that at the upper end closely conforms to the optical element and may be round, for example. At the bottom, the inner periphery closely conforms to the shape of the image field and may be rectangular, for example.

The first fluid 11 is confined to the reservoir by a gas seal 26 between the bottom of the seal member 21 and the surface of the transparent plate 12. The gas seal 26 is formed by gas, e.g. air or synthetic air, provided under pressure via inlet 25 to the gap between the seal member 12 and extracted via outlet 24. The overpressure on the gas inlet 25, vacuum level on the outlet 24 and geometry of the gap are arranged so that there is a high-velocity air flow inwards that confines the fluid. This shown in more detail in FIG. 5.

The gas seal 26 is formed by two annular grooves 28, 29 which are connected to the inlet 25 and outlet 24, respectively, by a series of small conducts spaced around the grooves. A large annular hollow in the seal member 21 may be provided in each of the inlet 25 and outlet 24 to form a manifold.

A gap G1, on the outer side of the gas inlet 25, is small and long so as to provide resistance to air flow outwards. A gap G2, at the radius of the inlet 25, is a little larger to ensure a sufficient distribution of gas around the seal member 21. The inlet 25 is formed by a number of small holes around the seal member 21. A gap G3 is chosen to control the gas flow through the seal. A gap G4 is larger, to provide a good distribution of vacuum. The outlet 24 is formed of a number of small holes in the same manner as the inlet 15. A gap G5 is small, to prevent gas bubbles entering the space, and in this embodiment to ensure that capillary action will always fill it with fluid.

The gas seal 26 is thus a balance between the capillary forces pulling fluid into the gap and the airflow pushing liquid out. As the gap widens from G5 to G4, the capillary forces decrease and the airflow increases so that the fluid boundary will lie in this region and be stable under the projection system PL.

The pressure difference between the inlet at the gap G2 and the outlet at the gap G4 as well as the size and geometry of the gap G3, determine the gas flow through the seal 26 and will be determined according to the specific embodiment. However, a possible advantage is achieved if the length of the gap G3 is short and the absolute pressure at the gap G2 is twice that at the gap G4, in which case the gas velocity will be the speed of sound in the gas and cannot rise any higher. A stable gas flow will therefore be achieved.

The gas outlet system can also be used to completely remove the fluid from the system by reducing the gas inlet pressure and allowing the fluid to enter the gap G4 and be sucked out by the vacuum system, which can easily be arranged to handle the fluid as well as the gas used to form the seal. Control of the pressure in the gas seal 26 can also be used to ensure a flow of fluid through the gap G5 so that fluid in this gap does not disturb the temperature of the fluid in the main space below the projection system.

The shape of the seal member 21 around the gas inlet 25 and outlet 24 should be chosen to provide laminar flow as far as possible so as to reduce turbulence and vibration. Also, the gas flow should be arranged so that the change in flow direction at the fluid interface is as large as possible to provide maximum force confining the fluid.

It should be appreciated that a similar system as shown in FIGS. 4 and 5 is between the substrate W and the transparent plate 12. The system shown in FIGS. 4 and 5 is more fully described in U.S. patent application Ser. No. 10/705,805, filed Nov. 12, 2003, the entire contents of which are incorporated herein by reference.

FIG. 6 shows a cover plate 27 provided on the substrate table WT. The cover plate 27 has an upper surface coplanar with the substrate W and is closely adjacent to the edge of the substrate W so that there is no sudden loss of fluid as the edge of the substrate W moves under the projection system PL. A vacuum outlet is preferably provided under the gap between the substrate W and cover plate 27 to remove any fluid passing into the gap and any gas from the gas seal to prevent it from entering the space.

A device manufacturing method includes providing the substrate W at least partially covered by a layer of radiation-sensitive material. The patterned beam PPB is projected onto a target portion of the layer of radiation-sensitive material using the projection system. The space between an optical element of the projection system and the substrate is filled with at least two fluids. The two fluids have different indices of refraction. The transparent plate separates the two fluids.

It will be appreciated that the device manufacturing method of the present invention may be preceded by design of the functions and performance of the device and a pattern to realize the functions. It will also be appreciated that the design and manufacture of a mask according to the present invention may also precede the method. It will also be appreciated that substrate, or wafer, production and processing may precede the method. The wafer processing may include, for example, oxidizing the wafer surface, forming an insulation film on the wafer surface, forming an electrode, such as by vacuum deposition, on the wafer, implanting ions into the wafer, and doping the wafer with a photosensitive agent. Other wafer processing steps that may follow the method include developing the resist, removing the developed resist, such as by etching, and removing unnecessary resist after etching. It will also be appreciated that device assembly and inspection, including, for example, dicing, bonding, packaging (chip sealing), and operation and durability check testing, may follow the method.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The description is not intended to limit the invention. 

1. A lithographic projection apparatus, comprising: a radiation system constructed and arranged to provide a beam of radiation; a support constructed and arranged to supporting a patterning device, the patterning device constructed and arranged to pattern the beam of radiation according to a desired pattern; a substrate table constructed and arranged to hold a substrate; a projection system constructed and arranged to project the patterned beam of radiation onto a target portion of the substrate; a transparent plate positioned between an optical element of the projection system and the substrate; a first fluid having a first index of refraction filling a first space between the substrate and the transparent plate; and a second fluid having a second index of refraction filling a second space between the transparent plate and the optical element, wherein the first and second indices of refraction are different.
 2. An apparatus according to claim 1, wherein the first index of refraction is greater than the second index of refraction.
 3. An apparatus according to claim 1, wherein the second index of refraction is greater than the first index of refraction.
 4. An apparatus according to claim 1, wherein the transparent plate has a third index of refraction.
 5. An apparatus according to claim 4, wherein the third index of refraction is between the first index of refraction and the second index of refraction.
 6. An apparatus according to claim 4, wherein the third index of refraction is substantially equal to the first index of refraction.
 7. An apparatus according to claim 4, wherein the third index of refraction is substantially equal to the second index of refraction.
 8. An apparatus according to claim 1, wherein the first index of refraction is substantially equal to an index of refraction of the substrate.
 9. An apparatus according to claim 1, wherein the second index of refraction is substantially equal to an index of refraction of the optical element.
 10. An apparatus according to claim 1, wherein the first fluid is one of a perfluoropolyether fluid and water and the second fluid is one of a perfluoropolyeter fluid and water.
 11. An apparatus according to claim 1, wherein the first and second fluids are perfluoropolyether fluids.
 12. A device manufacturing method, comprising: projecting a patterned beam of radiation onto a target portion of a layer of radiation-sensitive material at least partially covering a substrate; and filling a space between an optical element of a projection system and the substrate with first and second fluids having first and second indices of refraction, respectively, wherein the first and second indices of refraction are different.
 13. A device manufacturing method according to claim 12, further comprising: separating the first and second fluids with a transparent plate.
 14. A device manufacturing method according to claim 13, wherein a first space between the substrate and the transparent plate is filled with the first fluid and a second space between the transparent plate and the optical element is filled with the second fluid.
 15. A device manufacturing method according to claim 12, wherein the first index of refraction is greater than the second index of refraction.
 16. A device manufacturing method according to claim 12, wherein the second index of refraction is greater than the first index of refraction.
 17. A device manufacturing method according to claim 14, wherein the first index of refraction is substantially equal to an index of refraction of the substrate.
 18. A device manufacturing method according to claim 14, wherein the second index of refraction is substantially equal to an index of refraction of the optical element.
 19. A device manufacturing method according to claim 12, wherein the first fluid is one of a perfluoropolyether fluid and water and the second fluid is one of a perfluoropolyether fluid and water.
 20. A device manufacturing method according to claim 12, wherein the first and second fluids are perfluoropolyether fluids.
 21. A device manufacturing method according to claim 13, wherein the transparent plate has a third index of refraction between the first and second indices of refraction.
 22. A device manufacturing method according to claim 21, wherein the third index of refraction is substantially equal to the fist index of refraction.
 23. A device manufacturing method according to claim 21, wherein the third index of refraction is substantially equal to the second index of refraction. 